monero/contrib/epee/include/byte_slice.h

// Copyright (c) 2019, The Monero Project
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#pragma once

#include <cstddef>
#include <cstdint>
#include <memory>
#include <string>
#include <vector>

#include "span.h"

namespace epee
{
  struct byte_slice_data;

  struct release_byte_slice
  {
    void operator()(byte_slice_data*) const noexcept;
  };

  /*! Inspired by slices in golang. Storage is thread-safe reference counted,
      allowing for cheap copies or range selection on the bytes. The bytes
      owned by this class are always immutable.

      The functions `operator=`, `take_slice` and `remove_prefix` may alter the
      reference count for the backing store, which will invalidate pointers
      previously returned if the reference count is zero. Be careful about
      "caching" pointers in these circumstances. */
  class byte_slice
  {
    /* A custom reference count is used instead of shared_ptr because it allows
       for an allocation optimization for the span constructor. This also
       reduces the size of this class by one pointer. */
    std::unique_ptr<byte_slice_data, release_byte_slice> storage_;
    span<const std::uint8_t> portion_; // within storage_

    //! Internal use only; use to increase `storage` reference count.
    byte_slice(byte_slice_data* storage, span<const std::uint8_t> portion) noexcept;

    struct adapt_buffer{};

    template<typename T>
    explicit byte_slice(const adapt_buffer, T&& buffer);

  public:
    using value_type = std::uint8_t;
    using size_type = std::size_t;
    using difference_type = std::ptrdiff_t;
    using pointer = const std::uint8_t*;
    using const_pointer = const std::uint8_t*;
    using reference = std::uint8_t;
    using const_reference = std::uint8_t;
    using iterator = pointer;
    using const_iterator = const_pointer;

    //! Construct empty slice.
    byte_slice() noexcept
      : storage_(nullptr), portion_()
    {}

    //! Construct empty slice
    byte_slice(std::nullptr_t) noexcept
      : byte_slice()
    {}

    //! Scatter-gather (copy) multiple `sources` into a single allocated slice.
    explicit byte_slice(std::initializer_list<span<const std::uint8_t>> sources);

    //! Convert `buffer` into a slice using one allocation for shared count.
    explicit byte_slice(std::vector<std::uint8_t>&& buffer);

    //! Convert `buffer` into a slice using one allocation for shared count.
    explicit byte_slice(std::string&& buffer);

    byte_slice(byte_slice&& source) noexcept;
    ~byte_slice() noexcept = default;

    //! \note May invalidate previously retrieved pointers.
    byte_slice& operator=(byte_slice&&) noexcept;

    //! \return A shallow (cheap) copy of the data from `this` slice.
    byte_slice clone() const noexcept { return {storage_.get(), portion_}; }

    iterator begin() const noexcept { return portion_.begin(); }
    const_iterator cbegin() const noexcept { return portion_.begin(); }

    iterator end() const noexcept { return portion_.end(); }
    const_iterator cend() const noexcept { return portion_.end(); }

    bool empty() const noexcept { return storage_ == nullptr; }
    const std::uint8_t* data() const noexcept { return portion_.data(); }
    std::size_t size() const noexcept { return portion_.size(); }

    /*! Drop bytes from the beginning of `this` slice.

        \note May invalidate previously retrieved pointers.
        \post `this->size() = this->size() - std::min(this->size(), max_bytes)`
        \post `if (this->size() <= max_bytes) this->data() = nullptr`
        \return Number of bytes removed. */
    std::size_t remove_prefix(std::size_t max_bytes) noexcept;

    /*! "Take" bytes from the beginning of `this` slice.

        \note May invalidate previously retrieved pointers.
        \post `this->size() = this->size() - std::min(this->size(), max_bytes)`
        \post `if (this->size() <= max_bytes) this->data() = nullptr`
        \return Slice containing the bytes removed from `this` slice. */
    byte_slice take_slice(std::size_t max_bytes) noexcept;

    /*! Return a shallow (cheap) copy of a slice from `begin` and `end` offsets.

        \throw std::out_of_range If `end < begin`.
        \throw std::out_of_range If `size() < end`.
        \return Slice starting at `data() + begin` of size `end - begin`. */
    byte_slice get_slice(std::size_t begin, std::size_t end) const;
  };
} // epee